Re: [PATCH] dt-binding: arm/cpus.txt: fix dynamic-power-coefficient unit

[Punit Agrawal <punit.agrawal@arm.com>]

Hi Vincent,

Thanks for the patch. One comment about the choice of units below.

Vincent Guittot <vincent.guittot@linaro.org> writes:

> The unit of dynamic-power-coefficient is described as mW/MHz/uV^2 whereas
> its usage in the code assumes that unit is mW/GHz/V^2

Instead of choosing GHz as the base, I'd prefer to use uW/MHz/V^2. It'll
avoid introducing fractional GHz value for frequency calculations.

> In drivers/thermal/cpu_cooling.c, the code is :
>
> power = (u64)capacitance * freq_mhz * voltage_mv * voltage_mv;
> do_div(power, 1000000000);
>
> which can be summarized as :
> power (mW) = capacitance * freq_mhz/1000 * (voltage_mv/1000)^2

Which would then translate to -

power (mW) = power (uW) / 1000 = capacitance * freq_mhz * (voltage_mv/1000)^2

Thanks,
Punit

>
> Furthermore, if we test basic values like :
> voltage_mv = 1000mV = 1V
> freq_mhz = 1000Mhz = 1Ghz
>
> The minimum possible power, when dynamic-power-coefficient equals 1, will
> be :
> min power = 1 * 1000  * (1000000)^2 = 10^15 mW
> which is not realistic
>
> With the unit used by the code, the min power is
> min power =  1 * 1 * 1^2 = 1mW which is far more realistic
>
> Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org>
> ---
>  Documentation/devicetree/bindings/arm/cpus.txt | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/Documentation/devicetree/bindings/arm/cpus.txt b/Documentation/devicetree/bindings/arm/cpus.txt
> index 29e1dc5..0148d7d 100644
> --- a/Documentation/devicetree/bindings/arm/cpus.txt
> +++ b/Documentation/devicetree/bindings/arm/cpus.txt
> @@ -274,7 +274,7 @@ described below.
>  		Usage: optional
>  		Value type: <prop-encoded-array>
>  		Definition: A u32 value that represents the running time dynamic
> -			    power coefficient in units of mW/MHz/uV^2. The
> +			    power coefficient in units of mW/GHz/V^2. The
>  			    coefficient can either be calculated from power
>  			    measurements or derived by analysis.
>  
> @@ -285,7 +285,7 @@ described below.
>  
>  			    Pdyn = dynamic-power-coefficient * V^2 * f
>  
> -			    where voltage is in uV, frequency is in MHz.
> +			    where voltage is in V, frequency is in GHz.
>  
>  Example 1 (dual-cluster big.LITTLE system 32-bit):